[FEATURE] Custom MailTransport

Adds hook mechanism to the mailer API. Custom mail transporters can be
configured in the Install Tool.

Change-Id: Ib81eab61b657e07de37a8107df3dce3b96376f5c
Resolves: #24841
Releases: 4.7
Reviewed-on: http://review.typo3.org/7065
Reviewed-by: Georg Ringer
Reviewed-by: Dominik Mathern
Tested-by: Dominik Mathern
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter